Role of poly (A) tail as an identity element for mRNA nuclear export

Different RNA species are rigorously discriminated and exported by distinct export factors, but this discrimination mechanism remains largely unknown. We previously showed, by RNA microinjection experiments, that intronless mRNAs are discriminated from U snRNAs based on their difference in RNA length. However, it was unclear how they are discriminated in the natural situation in which their nascent transcripts emerge progressively during transcription. We hypothesized that transcription from the corresponding promoters is important for this discrimination. Here we show that contrary to our hypothesis, the discrimination process was not significantly influenced by whether transcription occurred from an mRNA- versus a U snRNA-type promoter. Rather, the features of transcribed RNAs determined the RNA identity, consistent with our previous results of RNA microinjection. Moreover, we found that the poly (A) tail can function as an identity element for mRNA export. The presence of a poly (A) tail of an appropriate length committed otherwise short Pol II transcripts to the mRNA export pathway in a dominant manner, indicating that the poly (A) tail either contributes to increasing the RNA length or functions as a platform to recruit mRNA export factors. Our results reveal a novel function of the poly (A) tail in mRNA export.